US Secretary of State John Kerry says Afghanistan is at a "critical moment" as he tries to resolve tensions over the disputed presidential poll.

Mr Kerry is in Kabul to meet both the candidates claiming victory in June's run-off.

During a hastily arranged visit, he is to have talks with Abdullah Abdullah and Ashraf Ghani.

At the start of his meeting with Mr Kerry, Mr Ghani said that he would agree to an "intensive audit" of votes.

Mr Ghani, who came out ahead in the run-off round, said that the audit would help ensure the "integrity and the legitimacy that the people of Afghanistan and the world will believe in".

Current President Hamid Karzai is stepping down after more than 10 years.

He took power following the US-led overthrow of the Taliban.

The US has been concerned at reports that Mr Abdullah, who official results suggest lost the election, is planning a "parallel government".

Analysis by Karen Allen, BBC News, Kabul

Secretary of State John Kerry's visit is "critical" in ensuring the election authorities deliver a credible result which is "broadly acceptable" to the Afghan people.

In mathematical terms that means pushing for a much broader audit of votes - beyond the 3 million currently identified. In diplomatic terms it means finding a way through the fog of mistrust so that both sides start working together.

When earlier this week Abdullah Abdullah faced pressure from his supporters to declare a parallel government, the US quickly responded by warning that such an act would trigger the suspension of aid and security assistance.

Mr Abdullah pulled back from the brink but he will have to show his frustrated followers that the meeting with John Kerry has broadened the scope of the audit to prevent them spilling onto the streets - angry and armed.

Ashraf Ghani will be looking for some lines in the sand. One of his close aides told me they would continue to co-operate with the election authorities but feared that the "cascading demands" of Abdullah's side - was simply a "delaying tactic. They're looking to John Kerry to manage that.

"We are at a very, very critical moment for Afghanistan," said Mr Kerry after meeting the head of the UN Assistance Mission Jan Kubis at the US embassy in Kabul.

"Our hopes are that there is a road that can be found that will provide that capacity for the questions to be answered, for people's doubts to be satisfied and hopefully for a future to be defined," Mr Kerry added.

The fact Mr Kerry made this trip at such short notice, is an indication of just how concerned the US is about the risk of Afghanistan descending into post-election turmoil, the BBC's Aleem Maqbool in Washington says.

It has become clear that seeing the way Iraq has unravelled so dramatically after the withdrawal of US troops has made the White House all the more desperate that Afghanistan should fare better, our correspondent adds.

Abdullah Abdullah, in the blue shirt, insists that he is the rightful election winner

Ashraf Ghani (centre) has called on his opponent to not "drag the country into further crises"

Taliban offensive

Preliminary results announced by Afghanistan's election officials give Mr Ghani 56.44% of votes in the 14 June run-off, with Mr Abdullah gaining 43.45%.

The results were markedly different from those achieved in the first round of voting, held in April.

In that round, Mr Abdullah fell just short of an outright majority, with 44.9%, with Mr Ghani second at 31.5%.

Both candidates have alleged fraud in the election. Votes are being re-checked at more than 7,000 polling stations - nearly a third of the total number.

Correspondents say recounts could significantly alter the final result, due on 22 July.

The UN Assistance Mission to Afghanistan has warned it will be "premature" for either side to claim victory.

It called on the presidential candidates to exercise restraint and prevent their supporters from "taking steps that could lead to civil disorder and instability".

There are also concerns about a further deterioration in the security situation.

Taliban militants have been testing the limits of the Afghan army in recent weeks, with a major offensive in the southern province of Helmand.

The withdrawal of foreign troops by the end of this year will be the litmus test of whether more than a decade of training and investment in building up Afghanistan's own security forces has paid off, correspondents say.

President Barack Obama has said the US remained committed to Afghanistan provided the incoming president signed a security agreement.

Both Mr Abdullah and Mr Ghani have said they are committed to signing the deal with the US that would allow a small force to stay on.

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